This invention relates to a monostable multivibrator formed of bipolar transistors.
Monostable multivibrators include those of a linear circuit type formed of bipolar transistors and those of a digital circuit type formed of MOSFETs (metal oxide semiconductor field effect transistor). Linear circuit type multivibrators have been generally used for a motor control circuit.
A conventional linear circuit type monostable multivibrator is shown in FIG. 1. In FIG. 1, a PNP multicollector transistor Q1 and an NPN transistor Q2 constitute a thyristor 1. Reference numeral 2 is a bias circuit or voltage divider for providing a reference bias voltage Vr to the base of transistor Q1. The bias circuit 2 comprises resistors R1 and R2, serially connected between a Vcc power source potential (positive potential) and a Vee power source potential (ground). The junction of resistors R1 and R2 is connected to the base of transistor Q1 and the collector of transistor Q2. A time constant circuit 3 comprises a resistor R and a capacitor C connected in series between Vcc and Vee power source potentials. The junction of resistor R and capacitor C is connected to the emitter of transistor Q1. An NPN transistor Q3 is for receiving a set signal. The base of transistor Q3 is connected to a set input terminal 4 and is supplied with a set signal. The emitter of transistor Q3 is connected to the Vee power source potential. The collector of transistor Q3 is connected to the base of transistor Q2 and one of the multicollectors of transistor Q1. An NPN transistor Q4 is used as an output buffer. The emitter of transistor Q4 is connected to the Vee power source potential. The collector of transistor Q4 is connected to an output terminal 5 of the whole vibrator circuit. The collector of transistor Q4 is also connected to the Vcc power source potential through a load resistor R.sub.L. The base of transistor Q4 is connected to another collector of transistor Q1.
The following is the operation of the prior art multivibrator as shown in FIG. 1. In a normal state, thyristor 1 is in the ON state. Now assume that thyristor 1 is in the ON state. At this time, transistor Q4 as an output buffer is ON, and an output voltage Vout of vibrator output terminal 5 is at a low level (substantially, ground potential). Now also assume that a high level (Vcc potential) set trigger signal Si (FIG. 2A) is input to the set input terminal 4. When the trigger signal is supplied to the terminal 4, transistor Q3 is turned momentarily ON to turn transistor Q2 OFF. Therefore, thyristor 1 is turned OFF. Thus, transistor Q4 is turned OFF, and the output voltage Vout (FIG. 2C) is changed to a high level. When thyristor 1 is turned OFF, an electric current starts to flow through time constant circuit 3 to charge capacitor C. When a predetermined period To of time has passed from the time when an electric current has started to flow through the time constant circuit 3 and a capacitor voltage Vc (FIG. 2B) across capacitor C exceeds the sum of bias voltage Vr and the emitter base voltage VbeQ1 of transistor Q1, thyristor 1 returns to the ON state and an output voltage Vout returns to the low level.
As clearly understood from the above, the pulse width of the output voltage Vout equals the time period To. The time To is determined by the reference bias voltage Vr, a base-emitter voltage VbeQ1 of transistor Q1 and a time constant defined by a capacitance of the capacitor C and a resistance of the resistor R of the time constant circuit 3. However, as well known, the emitter-base voltage VbeQ1 of transistor Q1 generally has a temperature dependence characteristic of -2 mV/.degree.C. Therefore, the time To, which is dependent of the emitter-base voltage VbeQ1, also has a temperature dependency characteristic. When, for example, transistor Q1 has a temperature dependency characteristic of -2 mV/.degree.C. and power source potential Vcc is 5 V, the capacitor voltage Vc varies 5 mV upon a temperature variation of 2.5.degree. C. This temperature variation is 0.1% of 5 V (power source potential Vcc). Therefore, when the monostable multivibrator is incorporated into a motor control circuit in a band held tape recorder set, the motor control circuit has also a temperature dependency characteristic. This lowers the waw characteristic of the motor control circuit.